Conventional interferometry measures the relative optical path difference between two arms (paths) in an interferometer. For example, in a Twyman-Green style interferometer having a reference arm where a beam of light is reflected from a reference object and a test arm where a beam of light is reflected from a test object an interferogram is formed by combining the two beams. The interferometer measures the optical path difference between the length of the test arm and the length of the reference arm by moving the test object and detecting the motion of the interference fringes produced thereby. The measurement measures the optical path difference as some multiple of the wavelength of light employed in the interferometer. The ambiguity in the range is removed by a priori knowledge of the actual distance of the test object to within a wavelength.
In absolute distance measuring interferometry, it is not necessary to know the absolute distance to the test object to within a wavelength of the light in the interferometer. The length of the test arm is measured by employing a plurality of frequencies of light. For example, the measurement may be accomplished by using a linear sweep or chripped frequency modulation of the light source. This produces a beat frequency on the output of the interferometer proportional to the distance to the test object and the modulation frequency. For example, see T. Kubota, et al., Optics Letters 12 (1987) 310-12. The problem with such absolute distance measurement interferometry employing linear frequency modulation is that to obtain accurate distance measurements, the test object must be at rest during the measurement. If the test object moves during the measurement, a Doppler frequency shift is produced in the interferometer resulting in an erroneous distance interpretation of the frequency at the output of the interferometer.
It is therefore an object of the invention to provide a method and apparatus for absolute distance measuring interferometry that is capable of measuring the distance to a moving test object.